Fender Hot Rod Deluxe to Single Channel AB763

[Willabe]

Board updated

And?????

[emerson909]

And.....

it's still under construction!

Got a busy weekend (actually going to 2 gigs!) so won't get it finished until next week, I know you're all as keen as I am to see if it works  Here's a another progress shot in the mean time.

[emerson909]

OK so I've finished rebuilding it and appears I've made some limited progress, the oscillation when the controls are maxed has stopped although I still have the rushing water/static noise. The voltages are as follows;

V1
Pin1: 187 Pin2: 0 Pin3: 1.73 Pin6: 187 Pin7: 0 Pin8: 1.75

V2
Pin1: 333 Pin2: 25mV Pin3: 4.82 Pin6: 333 Pin7: 25mV Pin8: 4.8

V3
Pin1: 194 Pin2: 0 Pin3: 1.62 Pin6: 195 Pin7: 240mV 0 Pin8: 1.61

V4
Pin1: 178 Pin2: 52 Pin3: 82 Pin6: 199 Pin7: 53 Pin8: 82

V5
Pin3: 440 Pin4: 438 Pin5: -41 Pin8: 17mV

V6
Pin3: 440 Pin4: 438 Pin5: -41 Pin8: 20mV

It looks to me like the noise is resulting from the 250mV DC on V3B grid, I measured 0V DC here on the initial build when the amp was working successfully.

I've tried lifting one leg of C17 as suggested by pdf64 but this makes no difference

I have observed that the voltage rises to around 500mV as I increase the reverb pot to maximum.

The voltage is still present if I remove V3.

I can't see where this voltage could be coming from other than a leaking coupling cap from V1B (C13) however I have tried swapping this cap previously with no change. I don't have any spares so will have to order a replacement, does anyone have any suggestions in the mean time?

I suspected the board but I thoroughly cleaned it with isopropyl when I rebuilt that section and I'm not measuring any dc voltage on the board around the terminals.

[pdf64]

...
It looks to me like the noise is resulting from the 250mV DC on V3B grid, I measured 0V DC here on the initial build when the amp was working successfully.

I've tried lifting one leg of C17 as suggested by pdf64 but this makes no difference

I have observed that the voltage rises to around 500mV as I increase the reverb pot to maximum.

The voltage is still present if I remove V3.
 ...

How about if the valves in V1 and V2 are removed?

If that doesn't help, how about if C13 (3n3F reverb recovery output) is lifted? 

[emerson909]

Just tried both suggestions and no change,

The only thing that stops the static noise is grounding V3B grid, I am totally stumped here 

DC resistance to ground from Pin7 is around 150K depending on the Reverb pot setting, I can't see how it can be getting a DC voltage.

I swapped V3 tube socket when I did the rebuild in case that was the fault, also tried several 12AX7s there as well but no change.

[emerson909]

Sorry misinterpreted your second suggestion there regarding the reverb recovery coupling cap, C13 is the V1B coupling cap, anyway I've just also lifted C19 the 3.3nF cap and no change.

So for clarity I've lifted C13, C17 and C19 and there is still 250mVDC present on Pin 7 in all three scenarios.

Only grounding Pin7 eliminates the static.

[stratomaster]

Forgive me, I haven't checked through the full thread, but do you have a grid leak on that triode (220k typical) or did go forgo that as in the Princeton Reverb?

If you disconnect the wire to the grid, do you still measure a voltage at the socket?  If so, and since you've ruled out tubes by swapping a few in, then you may need to look at the socket itself.

If you disconnect the grid wire and don't measure DC at the socket but do at the board then there's an off chance the black pigment in the fiberglass is causing it to be leaky.  This is provided there are no wiring errors or alternate paths for DC--so be sure you've ruled everything else out first.  Black in plastics (resins included) is typically derived from a carbon of some sort.  It even affects the mechanical properties of plastics compared to other colors.  It looks cool compared to the sickly standard color, but I'd be hesitant to use it in high voltage applications.  Not because of a track record of leakiness, but because of the increased possibility of it and the availability of alternatives without that risk.

[emerson909]

Yes it has a 220K grid leak (R38 on the schematic) which has continuity to ground.

If I disconnect the grid wire the static noise goes and I measured around 100mv on pin 7 which decayed to around 6mv over roughly 2 minutes. There is a loud hum while the meter is connected.

I measured 78mV at the terminal on the board.

You will have to forgive me but I honestly have no idea how to interpret this!! The colour of the fibre glass is however one of the things that has changed since it was working, you can see from the first pictures it was the standard green colour originally.

[stratomaster]

You will have to forgive me but I honestly have no idea how to interpret this!!

There are only two options left. 

• The coupling cap is leaky in a way that has eluded your efforts to identify it.
• The board itself is conductive enough to allow DC build up at the turret.

Try "floating" the connections to the grid instead of terminating everything at the turret/eyelet.  It's going to be messy and you'll have stuff sticking up in the air, but it's just for a temporary test.

If this fixes it, then I'd think the 2nd option is very likely.  The only way to confirm for sure is to use a "Megger" to measure the insulative integrity of the board. 

[emerson909]

Don't think I'll be able to get my hands on a Megger any time soon!

In hindsight there's not much clearance between some of the terminals, I've just doused the board a couple more times with isopropyl to try and get rid of any remaining residue, will let it dry out overnight and try again in the morning but not holding out much hope as it already looks pretty clean.

I'm still clinging on to the fact that it has worked previously and trying to think what could be different to before. I was also thinking about floating the connections but not sure how feasible/safe it would be.

Might have to be another redesign on a green board with extra spacing 

[Willabe]

I'm still clinging on to the fact that it has worked previously and trying to think what could be different to before.

Yes, but it only worked with band aid by-pass caps. It always had problems. 

[emerson909]

Yes, but it only worked with band aid by-pass caps. It always had problems. 

was referring to the DC voltage on V3B grid that wasn’t there originally.

[emerson909]

I've done a bit more trouble shooting this morning and can only conclude that there is DC leakage on the board.

I've disconnected C13 and C19 and measure around 500mV at the junction of R26/C17, I am also measuring this on what is now a spare terminal (out of circuit) alongside this junction. If I clip a lead from the disconnected side of C13 directly to V3 pin 7 the static noise goes and the grid voltage drops to a few millivolts.

What I was struggling to understand was if there is leakage via the board why is this only manifesting as an issue on this specific grid, i.e. surely I should see this on V3A then it occurred to me that V1 and V3A are connected to the pots/jacks directly so no connection via the board.

There are a few spare terminals on the board now as a result of the changes and I'm measuring various DC voltages to ground on all of them.

I obviously can't say if it's the board itself or something I have done to it but looks like the only solution is a rebuild on a new board.

Thanks for everyone who has taken an interest so far, really appreciate the help!

[pdf64]

Oh dear, that's unfortunate!
What material is the board, where did you buy it?
It looks a fibreglass type, so I thought this issue would be very unlikely, though it seems more common with the vulcanised compressed paper fibreboard traditionally used in vintage Fenders.

So, well done tracking the root cause issue down. 

[emerson909]

Yes it seems so unlikely that I had pretty much discounted it but I can't see any other explanation.

It's G10 fibreglass board bought from a reputable supplier here in the UK, I'm reluctant to say where because I may well be completely wrong given my limited diagnosis skills! I will reach out to them to see if they have had any other issues.

I have obviously done a lot of rework and whilst I have tried to clean the board the leakage may well be a result of my work somewhere!

[emerson909]

Well I'm very pleased to report that the supplier of the original board sent me a green replacement and after a rebuild the amp is working again, static oscillations and hum all gone!

I admit I've left all of the 'band aid' caps in place as I was working on the principle that it's much easier to install them during the build and remove them later but I'm going to enjoy the amp for a bit before I change anything, going to give the soldering iron a rest for a bit 

[emerson909]

Picture of the now working amp

[pdf64]

Well done for sticking with it, that was a weird bit of bad luck.